1. Field of the Invention
The present invention relates to a method of shutting down a fuel cell system, which includes a fuel cell for generating electricity from an electrochemical reaction between an oxygen-containing gas and a fuel gas.
2. Description of the Related Art
Fuel cells are used in a system for generating DC electric energy by supplying a fuel gas, i.e., a gas mainly containing hydrogen, e.g., hydrogen gas, and an oxygen-containing gas, i.e., a gas mainly containing oxygen, e.g., air, to an anode and a cathode, for thereby causing an electrochemical reaction between the fuel gas and the oxygen-containing gas. Such a system may be used as a stationary electric generating system, or may be mounted on fuel cell vehicles as an electric generating system.
Solid polymer electrolyte fuel cells, for example, incorporate a membrane electrode assembly (MEA) including an anode and a cathode disposed on respective sides of an electrolyte membrane, such as a polymer ion exchange membrane, and a pair of separators between which the membrane electrode assembly is sandwiched. A fuel gas flow field for supplying a fuel gas to the anode is defined between one of the separators and the membrane electrode assembly, and an oxygen-containing gas flow field for supplying an oxygen-containing gas to the cathode is defined between the other separator and the membrane electrode assembly.
When the fuel cell is shut down, supply of the fuel gas and the oxygen-containing gas is stopped. However, the fuel gas remains in the fuel gas flow field, and the oxygen-containing gas remains in the oxygen-containing gas flow field. If the fuel cell is shut down for a long period of time, the fuel gas and the oxygen-containing gas possibly may pass through the electrolyte membrane and mix with each other.
For example, according to Japanese Laid-Open Patent Publication No. 2004-022487, when a fuel cell is shut down, supply of the reactant gas to the cathode is cut off by an inlet cutoff valve, while the cathode outlet gas is circulated upstream of the fuel cell via a circulation line. The electrochemical reaction in the fuel cell continues to consume oxygen in the cathode outlet gas, and then an inactive gas, such as nitrogen gas, is used to purge the cathode and the anode of the fuel cell.
According to Japanese Laid-Open Patent Publication No. 2004-022487, an anode circulation line and a cathode circulation line, which circulate an anode outlet gas and a cathode outlet gas from the fuel cell to an anode gas supply line and a cathode gas supply line upstream of the fuel cell, are branched respectively from the anode gas supply line and the cathode gas supply line. The cathode circulation line is connected to a tank that serves as a gas volume region for storing nitrogen gas as an inactive gas.
As described above, the electrochemical reaction in the fuel cell continues to consume oxygen in the cathode outlet gas, and an inactive gas, such as a nitrogen gas, is used to purge the cathode and the anode of the fuel cell. Consequently, the fuel cell system is relatively complex in overall structure and large in overall size, and is highly costly to manufacture.